Individual semiconductor switching devices or elements, which have low ON resistance and exhibit a high-speed switching characteristic, include bipolar transistors and unipolar transistors. For example, insulated gate field-effect transistors (hereinafter abbreviated to "MOS transistor") are well known as an example of the unipolar transistors. Also, insulated gate bipolar transistors (hereinafter referred to as "IGBT") that are voltage-controlled bipolar transistors have been increasingly used in recent years. These semiconductor devices have the following features.
The bipolar transistor may use a high-resistivity substrate to form a collector layer, in order to ensure a high breakdown-voltage characteristic. Even with the use of such a high-resistivity substrate, the transistor of this type undergoes conductivity modulation based on injection of minority carriers while the device is used in a saturation mode, thus assuring reduced ON resistance. However, the turn-off time is increased due to the effect of accumulation of the minority carriers, resulting in a delay in switching the transistor.
The MOS transistor that is a unipolar transistor is inherently free from accumulation of minority carriers, and therefore operates at a high switching speed. This type of transistor, however, suffers from relatively large ON resistance since the conductivity modulation based on injection of minority carriers does not occur in this device.
Similarly to the bipolar transistor as described above, the IGBT exhibits relatively low ON resistance, but suffers from relatively long turn-off time due to the effect of accumulation of minority carriers. In addition, minority carriers are re-injected from the collector layer, due to electrons that are discharged by a depletion layer that spreads out during turn-off, which results in an increase in the turn-off time.
In a conventional switching circuit, therefore, bipolar transistors and IGBT, which have a small steady-state loss but suffer from a large switching loss, are often used at a relatively low frequency (generally, lower than 50 kHz), while MOS transistors, which have a small switching loss but suffers from a large steady-state loss, are often used at a relatively high frequency (generally, 100 kHz).
In an intermediate frequency region, namely, in the range of 20-100 kHz, it is desirable to use a semiconductor device having the characteristics of both of the above types of transistors, depending upon its applications. FIG. 51 shows one example of this type of semiconductor device as disclosed in Japanese Laid-open Patent Publication No. 62-293678. This example is a so-called BiMOS-Cascade transistor in which a unipolar transistor UT is provided in the front stage, and a bipolar transistor BT is provided in the rear stage. This semiconductor device exhibits low ON resistance, and can be controlled by voltage, but suffers from a problem that the turn-off time, in particular, storage time, is considerably increased because an excessive amount of carriers stored in the bipolar transistor BT during turn-off cannot be removed or drawn away from the transistor BT.
To overcome this problem, there has been proposed a semiconductor device as shown in FIG. 52, wherein a resistor R is provided between the base and emitter of the bipolar transistor BT. Examples of this type of semiconductor device are disclosed in Parpia, Z. et al., IEEE Trans. on ED, vol.33, 1984 (1986), ibid. vol.35, 1687, (1988), Narayanan, S. et al., IEEE ISPSD '91, 103, (1991), Narayanan, S. et al., IEEE Trans on ED, 1624, (1991), Godignon, P. et al., MADEP, 0-215, (1991), Ajit, J. S. MADEP, 0-148, (1991), Godignon, P. et al., Solid-state Electronics, 1777, (1996). In this type of semiconductor device, carriers may be drawn away from the base region of the bipolar transistor BT through the resistor R during turn-off, and therefore the turn-off time can be shortened. However, the ON resistance is somewhat increased since current flows through the resistor R during turn-on as well as turnoff. As another method for solving the problem of increased turn-off time, a semiconductor device as shown in FIG. 53 has been proposed in Japanese Patent No. 3-66816, wherein a diode D is connected between the base of a bipolar transistor BT and the gate of a unipolar transistor UT. During turn-off of this semiconductor device, carriers can be drawn from the base region of the bipolar transistor BT into the gate through the diode D, and therefore the turn-off time can be shortened. However, a small amount of carriers still remain due to a barrier-layer voltage at the junction of the diode D.